Engines are usually designed with the ability to deliver a peak output, although most engine operation is performed well below this peak value. As such, it can be beneficial to operate with some cylinders inducting air without fuel injection as described in U.S. Pat. No. 6,568,177.
As described in U.S. Pat. No. 6,568,177, periodically it may be necessary to purge stored NOx by operating all of the cylinders stoichiometric or rich.
The inventors herein have recognized that such an approach may result in wasting excess fuel in configurations where there are multiple cylinder banks, each having an upstream emission control device, when it is desired to reduce NOx stored in a downstream device. Specifically, fuel is must be used to reduce stored oxidants in both upstream devices, before downstream stored NOx can be reduced.
The above disadvantage can be overcome by a method for controlling an engine having at least a first and second group of cylinders. The method comprises:
operating the engine in a first mode wherein the first cylinder group operates with air and substantially no injected fuel and the second cylinder group operates to combust air and injected fuel; and
transitioning the engine to operating in a second mode wherein both the first cylinder group and the second cylinder group operate to combust air and injected fuel, combustion in one of said first and second cylinder group oscillating about stoichiometry and combustion in the other of said first and second cylinder group being rich of stoichiometry
In this way, it is possible, in one embodiment, to provide the ability to operate some cylinders without fuel injection, and save still further fuel when it may be desired to purge NOx stored in a downstream emission control device. Note, however, that the above approach can be used in other situations as well to improve performance.
In another aspect, another disadvantage with prior approaches can relate to transitioning from operation with some cylinder deactivated and active cylinders combusting, to all cylinders combusting. Specifically, retarded ignition timing is used, along with throttling to reduce any torque disturbance that may result. However, throttle is generally to slow due to manifold filling effects, and as such, ignition timing retard is heavily relied upon. Significant reliance on ignition timing retard, however, can result is degraded fuel economy.
Therefore, to overcome the above disadvantage, the inventors herein have recognized that the above fuel economy degradation can be reduced by operating the re-enabled cylinders at a lean combustion limit, thereby reducing the amount of ignition timing retard that may be needed. In one example, a method for controlling an engine having at least a first and second group of cylinders comprises:
operating the engine in a first mode wherein the first cylinder group operates with air and substantially no injected fuel and the second cylinder group operates to combust air and injected fuel; and
transitioning the engine to operating in a second mode wherein both the first cylinder group and the second cylinder group operate to combust air and injected fuel, combustion in both said first and second cylinder group being lean of stoichiometry at a lean combustion limit with retarded ignition timing.